A functional coating or one or more functional coatings can be applied to a surface of article in order to impart one or more properties or characteristics to the surface of the article. Such properties or characteristics imparted may include color, gloss, reflectivity, abrasion resistance, optical clarity, water repellency, resistance to fogging, anti-reflectivity, resistance to soiling, and ease of cleaning. Of these various properties, the surface property or characteristic of ease of cleaning or easy-cleaning has potentially broad applications across a variety of industries including optical and non-optical related industries.
Current technologies employed to produce easily cleanable coatings or properties on surfaces often involve the application of a coating or surface treatment that is both hydrophobic and oleophobic. It has conventionally been considered desirable that a functional coating repel both oils and water, hence, these characteristics have been held as the benchmark for cleanability in the marketplace. Unfortunately, basic market surveys reveal that current easy-cleaning coating technologies do not perform at the levels expected by the market or the level indicated by marketing literature.
Current easy-cleaning coating technologies are chiefly directed towards achieving the highest possible wetting angle for both water and oils because correlations have been made between high wetting angle for water and oil and surface resistance to smudges, fingerprints, and ease of cleaning. The basis of this target wetting performance lends to the objective that current easy-cleaning coating surface energy be rendered as low as possible, e.g. below that of Teflon-like materials or below approximately 17-21 millijoules per square meter. In general, coatings of these types do not perform well with respect to cleaning because complete non-wetting of oily substances has not been achieved in a durable and manufacturable coating system. Instead, the inherent partial wetting of oily substances on current coatings actually enhances the smeared appearance created when a thin layer of oils or dirt is left behind and is present as small droplets after cleaning is attempted. The presence of tiny droplets creates a hazy appearance by way of light scattering.
In optical-based industries, it is often desirable to combine the properties or characteristics of ease of cleaning with other functional properties or characteristics, for example, anti-fogging. However, the inherent hydrophobicity of current easy-cleaning coatings renders them not feasible for fogging resistance since water droplets are capable of adhering to them in a partially non-wetted state, creating the appearance of fogging by way of light scattering.
Current anti-fogging coatings utilize a different technology when compared to typical cleanable coating systems. Since the mechanism of fogging involves the formation of small water droplets that scatter visible light, coating systems that force the water to completely wet the surface eliminate this effect. The majority of anti-fogging technologies rely on the deposition of a surfactant on the surface of the article, which results in a short-lived resistance to fogging since the layer is quite temporary under conditions of use. Most often, the act of cleaning the surface is sufficient to remove the anti-fogging property.
Other technologies employ the use of a hydrophilic coating layer, usually composed of a material that has a permanent hydrophilicity. However, these types of coatings are typically not durable and cannot be used in applications where abrasion is present. In addition, commercially available hydrophilic anti-fogging coatings do not necessarily lend themselves to being cleanable, based on their inherent surface energetic properties. Accordingly, current coatings are marketed as either easy to clean or having anti-fogging properties. This is a result of the non-existence of a coating system that combines both properties in a durable and manufacturable way.
In optical-based industries, it is often also desirable to combine the properties or characteristics of ease of cleaning with functional properties or characteristics such as the property of anti-reflectivity. For example, current easy-to-clean, anti-reflective coatings for use on, for example ophthalmic products, typically consist of a first substrate, e.g. an uncoated lens or other substrate material, an optional hard-coating system, and an anti-reflective coating system, which is imparted with a hydrophobic or superhydrophobic top or outermost layer intended to provide an easy-cleaning property. This top-functional coating typically comprises silica and perfluorosilyl chemical groups, which impart a dewetting behavior of water and oil-like substances. The dewetting of the oil-like substances from this surface having a low apparent surface energy is what is currently considered to be the easy-cleaning property marketed and/or tested using current ophthalmic industry standardized testing. Such easy-cleaning, anti-reflective systems or coatings fail to perform at the levels expected by the market or the level indicated by marketing literature.
Hence, there exists a need for coatings and coating systems that impart effective easy-cleaning properties to a surface of an article alone and in combination with other functional coatings or coating systems that impart desirable surface properties.